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The mechanics of particle motion in a fluidized bed were studied by focusing on the microscopic behavior of the particles. The local displacements of nylon particles fluidized by liquid were measured by the microcapacitance method, and the resultant time series was analyzed by determining its auto-correlation function and the corresponding power spectrum. This has given rise to a stochastic or statistical model of particle displacements in the fluidized bed. This model visualizes the particle motion in a fluidized bed to consist of the random movement, generating irregular signals, and the linear movement, generating wave-like signals. Recently, Yutani and Ototake (1980) analyzed the motion of particles in a liquidsolids fluidized bed by a statistical method. To enhance the theoretical basis of their approach, additional microscopic measurements need to be made, and a mechanistic model for the particle motion needs to be developed. N. YUTANIIn this study, local displacements of nylon particles fluidized by a fluidizing medium were measured by the micro-capacitance method. The resultant time series were analyzed by determining their autocorrelation function and power spectrum. An autocorrelation function gives rise to the variance of particle displacements and the relaxation time of particle movement in the field; it can provide useful information for estimating the local particle diffusivity which is of practical importance in understanding mechanism and in determining the mass or heat transfer between the fluid and particles in a fluidized bed. The power spectrum yields information related to the local velocity distribution of particles. CONCLUSIONS AND SIGNIFICANCEA new technique, which utilizes a micro-capacitance probe, has been developed to measure local fluctuations of particle motion in a liquid-solids fluidized bed. The results indicate that the recorded signals are composed of a sine wave component and a stochastic component. A model has been proposed by taking these two components into account, and the auto-correlation function of the model has been fitted to experimental data by adjusting the frequency of the sine wave, fo, and the intensity of transition of particles, A. In addition, fo/A has been found to be independent of the void fraction of the fluidized bed, c, but dependent on the viscosity of the fluidizing medium, p. The power spectral density function derived from the model has been compared with that from the data. Although a liquid-solids fluidized bed is used in this study, there is no reason to suspect that the methodology developed here can not be applied to a gas-solids fluidized bed. In fact, it is easier to use the microcapacitance probe in the latter than in the former because the probe need not be insulated in the gas-solids bed. THEORETICAL Mechanism of Particle MotionExperimental observations (e.g., Houghton, 1966;Carlos and Richardson, 1968) of the behavior of particles in liquid-solids fluidized beds have revealed that flow of the fluidizing medium induces not only convective fl...